1. Field of the Invention
The present invention relates to an ultrasonic welding device for transverse welding in a flowpack packaging machine.
2. Description of Prior Art
Flowpack packaging machines form a continuous chain of package blanks movable in a longitudinal direction. The continuous chain of package blanks is formed by a continuous succession of products, spaced apart in the longitudinal direction, and wrapped in a tubular packaging film. A longitudinal welding device performs a continuous weld between two facing edges of the tubular packaging film along a direction parallel to the longitudinal direction along which the continuous chain of package blanks advances.
Downstream of the longitudinal welding device, a transverse welding device is provided, which carries out the welding of the tubular packaging film in a transverse direction relative to the feed direction. The transverse welds are spaced apart from each other by a length equal to the length of the individual packages. Generally, the transverse welding device also performs the transverse cutting of the packaging film between two adjacent welds forming the tail weld of a previous package and the head weld of a subsequent package.
The document EP-A-1421000 describes a flowpack packaging machine equipped with an ultrasonic longitudinal welding unit and an ultrasonic transverse welding unit. The ultrasonic transverse welding unit comprises a rotating sonotrode unit having at least two sonotrodes and a rotary anvil having at least two welding anvils. The sonotrode unit and the rotating anvil rotate about respective transverse axes relative to the direction of movement of the continuous chain of package blanks.
The rotating sonotrode is subject to an ultrasonic radial vibration, normally with a frequency between 20 and 50 kHz, which is manifested on the surface of the sonotrode facing a welding edge of the anvil. The vibration of the active surface of the sonotrode has variable amplitudes, approximately between 10 and 50 microns, and can be varied by adjusting the power of the ultrasonic generator.
While the longitudinal welding is carried out continuously over the entire length of the packaging film, in the transverse welding, the sonotrode-anvil contact with the packaging film occurs alternately between one product and the next, since it must allow the passage of the products through the transverse welding group between the successive welding lines.
For the execution of the transverse welding, the welding power is applied on a generatrix of a cylinder for the entire width of the tubular packaging film to be welded. The welding area is normally limited to a few millimeters, but the application of the welding power must take place simultaneously on the entire width of the welding line.
To obtain a good transverse welding result, a high power therefore needs to be used. This implies that in the initial point of contact, breakages can be created due to an excess of power or vibration amplitude.